The existence of composite piezoelectric transducers is quite recent. Composite piezoelectric ceramics are constituted by conventional PZT (zirconate/titanate) type piezoelectric elements included in a polymer matrix. For a detailed description of the technological principles involved in implementation, reference should be made to an article by Wallace Arden Smith published at pp. 755-766 of IEEE Ultrasonic Symposium, 1989.
The use of composite materials for making piezoelectric ceramics turns out to be advantageous since an acoustic impedance is obtained that is close to the acoustic impedance of biological tissues, and this translates into an increase in the sensitivity of the transducer and to a shorter impulse response. This enables such transducers to be used in medical echographic probes with the advantage of increasing the spatial resolution of such echographic probes.
This manufacturing technology also makes it possible to manufacture complex geometrical shapes, e.g. merely by thermoforming plane elements. Also, the use of a composite makes it possible to achieve transverse coupling that is very low such that the displacement of the sound-emitting surfaces is mainly unidirectional. This contributes to improving the electromechanical coupling and when the transducers are bar-shaped, it limits interactions between them, and this feature is particularly looked-for in echographic probes. It is also possible with composite materials to implement bar type transducers merely by integrating electrodes of the desired shape without cutting up piezoelectric elements.
As a result, composite materials are well-adapted to implementing medical echographic probes and to non-destructive monitoring where requirements are essentially centered on improving electromechanical coupling and spatial resolution for very low emission powers.
Heretofore, it has been believed that composite materials are not suitable for use in therapy (which requires high energy levels) because they suffer from a drop in performance at high excitation levels, as emphasized by Wallace Arden Smith in the above-specified article at p. 758. Mr. Smith emphasizes that at high energy levels transverse coupling increases and the piezoelectric element loses the unidirectional nature of its motion by acquiring transverse deformations that would make it impossible to use bar-type transducers, for example, even in medical echography. Further, firstly because of the increased transverse coupling and secondly because of the addition of polymer material, non-linear behavior of the transducer at high power is suggested.
Under such conditions, the person skilled in the art has had a prejudice against using composite piezoelectric elements for therapeutic treatment by means of ultrasound because high energy levels are required for that purpose.
In contrast, the present inventors have just discovered in a totally unexpected manner that composite piezoelectric elements are strong enough to enable them to emit sufficient energy to achieve therapeutic treatment, be that lithotrity (i.e. by means of a shock wave focused on a focus or target point), or for destroying or treating cells or tissue of a living being by the thermal effect that is obtained from focused ultrasound.
It is recalled here that the energy or power levels required for medical therapy are much greater than those needed for medical imaging. For example, for lithotrity, hyperthermia or tissue removal, the (pulse) energy levels are more than 10,000 times greater than those needed by medical imaging. In terms of power per pulse, these same levels are from 10 times to 1,000 times greater. For example, the energy level that needs to be transmitted for medical imaging is typically less than 1 ten thousandth of a J per pulse. For therapeutic treatment, the energy that needs to be transmitted by the piezoelectric elements is significantly greater than 0.01 J per pulse. For lithotrity the energy level required generally lies in the range 0.01 J per pulse to 1 J per pulse. For medical hyperthermia and for thermal tissue removal, the energy required is generally in the order of several tens of Joules.